Man Finds Galaxies

Man Finds Galaxies

Debi Prasad Choudhary

11/2/2014

In pursuit of knowledge, I reached a stage where none of my childhood experience related. The creation is vast beyond what we can see with our naked eye and comprehend with a single mind. The only thing relevant from my early learning that made sense is that my father’s teaching, “the creation is beyond our imagination, it contains smaller than the smallest and bigger than the biggest”, adopted from Hindu scripture Bhagabat Gita.

In first two decades of twentieth century, long exposure photographs of the sky with larger telescopes showed nebulous, fuzzy objects apart from stars. Some of them were spiral, some elliptical and some irregular shape. When such images of galaxies were put together, it seemed that there is a systematic transition from one shape to the other. The classification could be made from galaxies with tighter spiral arms, more dust and gas to the ones that had loose elliptical shape with less dust and gas. Most galaxies are found in groups bound by gravitational attraction. The spiral galaxies are loosely bound with members of few dozens. The elliptical galaxy groups contain hundreds and thousands of galaxies making huge groups. As the galaxies move around they collide and the interaction distorts their shape.

In the beginning, it was not clear if these nebulous objects resided inside the Milky Way or out side.  There was a great debate on April 26, 1920 between two famous astronomers Shapley and Curtis, who took opposite side and declared victory!! The key to answer this question is to measure distance to these objects. Several methods have been devised to determine the distance to the galaxies; they are based on the fact that when a candle is located at twice the distance it appears four times fainter. It is called “inverse square law”. There are many objects in the sky for which we know their brightness precisely. For example, the stars that are illuminated due to nuclear fusion at their center (called main-sequence stars), the supernova of white dwarf stars and Cepheid variable stars. Two astronomers, Tully and Fisher found that the brighter galaxies rotate faster. So, if we can identify a star of a particular type in a galaxy or determine the rotation speed of a galaxy, we can measure its brightness. The distances to these galaxies are measured by comparing its measured brightness with the expected brightness.

The distances to the galaxies are so large that light takes several million, in many case billions of years to reach us. There is no way to know what is happening to these galaxies now. Further away the galaxy is, we see what it was in the deep past. So, actually we see the history of the universe by studying the galaxies of the past. We now know that the spiral galaxies are made only in past 5 billion years, before which there were mostly irregular shaped galaxies. In the past, galaxies were colliding and merging into each other frequently, and producing much more stars in bursts.

Measuring the distance to the galaxies and the speed at which they move relative to us, Edwin Hubble made a fantastic discovery in 1929. He found that all the galaxies, except four are moving away from us. The galaxies that are located at greater distance move faster away from us. The four galaxies that are coming towards us are in a local group that moves randomly.  He measured the recession velocity of the galaxies by determining the difference of wavelength of spectral lines emitted by an atom in the galaxy and a similar atom on earth. This is known as Hubble’s law that describes the expansion of the universe. The universe, in which we are living, is expanding, all galaxies are moving away from each other!!!!

This is fantastic. Because, this means there was beginning when all were together. When was that? How close were they? How did it happen and why? These are profound questions. All that we know now is that all these happened at least about 13.6 billion years ago if the universe is always expanding at the present rate.